Speckle pattern inferometry (SPI) is a useful technique for evaluation of a vibrating surface. In this regard, inspection of a vibrating surface signifies a procedure used to analyze structure connected with a surface under actual or simulated conditions of use. For example, normal modes of oscillation of a structure can be determined by observing the vibrating surface of the structure.
The art of electronic speckle pattern interferometry (ESPI) has developed to provide a means and method for translating mechanical vibrations of an oscillating object surface into electronic signals representing an image of the surface. In this procedure, a first beam of light is generated by reflection from the vibrating surface to be analyzed. Because the object surface is irregular, the scale size of the irregularities being on the order of the light wavelength or greater, a random interference pattern known as "speckle" is produced in the image plane by the first beam. Each speckle is generated by light reflected from a localized area of the object surface. The speckles in the interference pattern are not unlike the speckles produced when a visible beam of laser light reflected from an uneven surface is observed by the human eye. This speckle pattern is then mixed with a second beam, the interference pattern created by this mixing being transformed, at an image plane, into an electronic signal representative of the interference image.
The vibrations of the object surface will dynamically deform, or contour, the surface. Consequently, the optical wavefront reflected from the surface will experience a Doppler shift in frequency. When the reflected beam is mixed with the second beam to form an interference pattern, the Doppler shifts in the wavefront of the first beam produce intensity variations in each image plane speckle, the frequency of these variations corresponding to the amount of Doppler shift inflicted on the reflected beam by a localized vibrational deformation on the object surface.
The speckles (variations in intensity, or brightness) in the interference image are converted into an electronic image. Typically, the electronic image has the form of a standard television signal, which includes a succession of video image frames. Each video image frame is a complete "snap shot" representative of the instantaneous deformation of the object surface at the time the image frame was formed. The succession of instantaneous snap shots is presented visually on a TV camera for viewing. In the prior art, the image of the object surface is processed by combination of successive image frames, which may be, for example, added or subtracted to emphasize or enhance certain features of the image.
In the art, Macovski in his U.S. Pat. No. 3,649,754 establishes the basic complement of elements for a speckle pattern interferometer employed to inspect a dynamically altered object surface. The basic holographic technique and apparatus of the Macovski is elaborated in U.S. Pat. No. 3,816,649 of Butters et al., in which a relationship geometry between the first and second beam was established to limit the range of spatial frequencies in the speckle pattern of the interference image. In Leendertz, U.S. Pat. No. 4,018,531, improvements in the optics used to form the second beam are taught, which result in the second beam satisfying the geometrical relationship of the Butters patent. In Pollard's U.S. Pat. No. 4,191,476, the basic Macovski/Butters speckle pattern interferometer is further improved by using a single set of optics to conduct light of two different wavelengths, to form two respective interference images at the same image plane.
Those skilled in the art will understand that the application of speckle pattern interferometry as currently applied in the inspection sciences is normally limited to apparatus which mechanically isolate both the surface being inspected and the electro-optical system from ambient vibrations induced by the local environment. Such isolation is achievable only through the employment of extreme measures, such as optical tables with elaborate suspension systems. Stability of the optical system is paramount, and should be on the order a fraction of an optical wavelength. The inventor has summarized the efforts in the art to stabilized the optical component of a speckle pattern interferometer in the APPLIED OPTICS article entitled "Optically phase-locked electronic speckle pattern interferometer", V. 26, No. 3, 1 Feb. 1987, Moran et al. Other efforts directted to optical stabilization of prior art speckle pattern interferometers either employ techniques which inherently limit improvements in displacement sensitivity, or which require prior knowledge of the vibrational characteristics of the object surface. Consequently, the prior art speckle pattern interferometer generally remains a laboratory instrument, unsuited to in situ inspection.
A principal objective of the inventor has been to define the basic array of elements necessary to provide an optical speckle pattern interferometer shorn of sensitivity to the environoment in which it operates.
Another object of the subject invention is to provide an optical speckle pattern interferometer with features which optically lock the instrument's operation to the dynamic characteristics of a surface being inspected, without prior knowledge of those characteristics, thereby permitting the instrument to distinguish those characteristics from other dynamic environomental characteristics of no interest to the inspection.
A principal advantage of the optical speckle pattern interferometer of the invention is its inherent portability, in contrast to the non-portability of the "optical table" configurations of the prior art speckle pattern interferometers.